Method of controlling a dc-to-dc converter

ABSTRACT

A HIGH-POWER DC-TO-DC CONVERTER (OR &#34;CHOPPER&#34;) SYSTEM PROVIDES AN INPUT VOLTAGE V1 OF SUBSTANTIALLY CONSTANT LEVEL TO A CHOPPER WHICH INCLUDES A SWITCH. THE OPENED AND CLOSED TIMES OF THE SWITCH ARE CONTROLLED TO PASS AN OUTPUT VOLTAGE VO OF VARIABLE MAGNITUDE THROUGH A FILTER TO A LOAD. A CONTROL CIRCUIT IS CONNECTED TO ACTUATE THE CHOPPER OVER A VARIABLE FREQUENCY RANGE, AND PRACTICAL OPERATING CONDITIONS SUCH AS HEATING OF THE SWITCH IMPOSE AN UPPER LIMIT ON THIS FREQUENCY RANGE. THE CHOPPER IS OPERATED SUCH THAT THE OUTPUT VOLTAGE VO IS ONE-HALF THE INPUT VOLTAGE VI AT THE MAXIMUM OPERATING FREQUENCY OF THE CHOPPER. TO PROVIDE LEVELS OF OUTPUT VOLTAGE VO LESS THAN HALF VI, THE FREQUENCY IS GRADUALLY REDUCED AS THE CHOPPER ON DWELL TIME IS MAINTAINED APPROXIMATELY CONSTANT OR ALLOWED TO DECREASE. TO PROVIDE LEVELS OF OUTPUT VOLTAGE VO GREATER THAN ONE-HALF THE INPUT VOLTAGE VI, THE CHOPPER OPERATING FREQUENCY IS GRADUALLY REDUCED AS THE CHOPPER OFF DWELL TIME IS MAINTAINED SUBSTANTIALLY CONSTANT OR ALLOWED TO DECREASE. CIRCUIT DETAILS OF A CONTROL CIRCUIT FOR OPERATING THE CHOPPER ACCORDING TO THIS METHOD ARE SET OUT.

Jan. 2 6, 1971 G, H, STUDTMANN lET AL v I 3,559,028

METHOD OF CONTROLLING A DC-TO-DC. CONVERTER 9 Sheets-Sheet 1 Filed Nov. l5, 1968 @2 J hopperQO 31T! YO Lood Ato eff' offTime (T-A) [3k/- J f I METHOD OF CONTROLLING A DG-TO-DC CONVERTER I Filed Nov. l5, 1968 G. H. STUDTMANN ET AL Jan. 26, 1971 9 Sheets-Sheet 2 Alie E y m A o` Nm A fnven'rors George H. Studtmonn Raymond J. Yoremo By JW V m. W @m mmc, C H .w- ^m.o X mi? x Cw. P w o E c 0| l w D. S v n E025 Y ErTro E225@ Y Q. o .A 6 o/ w rney Jan. 26, 1971 G. H. sTuDTMANN ET AL 3,559,023

METHOD oF CONTROLLING A Down-Dc CONVERTER Filed Nov. 15, 1968 9 Sheets-Sheet 3 220 5 @c @E025 E595 @E o m2o 952mm d 1D C 1G n /O. /O /O /O num m flu flu G mm@ Ll Ll LI VIH LmUvnlu r r T Lvo F TVO' mSJ VOI-L Hd n o GO V|V mm W wm, GRM VJ B me m nrl T. w Mm T. A@ AH A A T A d Jan. 26, 1971 G. H. STUDTMANN ET Al.

METHOD OF CONTROLLING A DCTODC CONVERTER Filed Nov. l5y, 1968 9 Sheets-Sheet 4 Raymond J. Yorema At rney Jan. 26, 1971 G. H. STUDTMANN' ET AL 'A 3,559,023

METHOD ,0F CONTROLLING A o o-Tomo CONVERTER Filed Nov. 15, 196e e sheets-sheet e t3* H611t V" T v0 FI i G. l la i puise-Wam 1Q Mgdg-,r'on 37 Fixed Pulse-Width variableu y /Frequency Modulation System .5 Variable Frequency D Modulation System DE E 1- 'gw O Typical Curve for Hybrid 5 Modulation System g 55 Hybrid Modulation System mentors 6 (Ronge of Cu'ves) George H. Studtmann Raymond J. Yarema o oVi 1.o\/1 Average Chopper Output Voltage By *w-v l J V0 Attorney Jan. 26, 1 971 Filed Nov. 15, 1968 G.v H. sTuDTMANN ETAL METHOD OF CONTROLLING A DC-TO-DC CONVERTER 9 Sheets-Sheet '7 Sensing 8i Clamping Cirouir Pulse Circuii Sensing 8i Clamping Circuii Pulse Circuif Inventors George H. Siud-imann Raymond J. Yarema By JM,

Ai'fo'rney Jan. 26,- 1971 G. H. STUDTMANN ET AL' 3,559,028

METHOD 0E CONTROLLING A nc-To-Ec CONVERTER Filed Nov.'15, 196e 9 Sheets-Sheet 8 H L M\ Q @maw @NQS w@ @Q oN 1\ mi o@ E :i im Wm t 1 M l@ A. m@ E bl. mm L* A? MQ 2 w@ (Si I h 1 Inventors O r mw .f .004, A UY .d Hn Em mm, e GREVE,

EE E

Jan. 26, 1971 G. H, STUDTMANN ErAL y 3,559,028

`METHOD OF CONTROLLING A DC-TO-DC CONVERTER Filed Nov. l15, 196s 9 sheets-sheet 9- T I H617@ t1 Tte 1 "*2 +11 I +11 Vp Vp ,v1 I /I I H617@ v Isa A +12 *+12 FIG. v17d Vp Vp V2 O/ H617@ +\/X +\/X V002@ FIG. 171E +Vx +VX VC'UBQDI FIG. 17g

nven'rors George H. Sudrmonn Raymond J. Yorema BYJLM l United States Patent O 3,559,028 METHOD OF CONTROLLING A DC-TO-DC CONVERTER George H. Studtmann, Mount Prospect, and Raymond J. Yarema, La GrangeA Park, Ill., assignors to Borg-Warner Corporation, Chicago, Ill., a corporation of Delaware Filed Nov. 1S, 1968, Ser. No. 776,109 Int. 'CL H02m 3/22 U.S. Cl. 321-2 8 Claims ABSTRACT OF THE DISCLOSURE A high-power DC-to-DC converter (or chopper) system provides an input voltage V1 of substantially constant level to a chopper which includes a switch. The opened and closed times of the switch are controlled to pass an output voltage Vo of variable magnitude through a lter to a load. A control circuit is connected to actuate the chopper over a variable frequency range, and practical operating conditions such as heating of the switch impose an upper limit on this frequency range. The chopper is operated such that the output voltage Vo is one-half the input voltage V1 at the maximum operating frequency of the chopper. To provide levels of output voltage Vo less than half V1, the frequency is gradually reduced as the chopper on dwell time is maintained approximately constant or allowed to decrease. To provide levels of output voltage V greater than one-half the input voltage Vi, the chopper operating frequency is gradually reduced as the chopper otf dwell time is maintained substantially constant or allowed to decrease. Circuit details of a control circuit for operating the chopper according to this method are set out.

BACKGROUND OF THE INVENTION A chopper or DC-to-DC converter in general comprises a switching circuit supplied with a DC bus voltage V, and operable to provide a variable DC output voltage Vo which is a fraction of the input voltage level V1. This is accomplished by actuating a switch, frequently a semiconductor switch such as a silicon controlled rectifier (SCR) or a power transistor, to apply either virtually the full DC bus voltage V1 or substantially zero voltage to a filter which is coupled between the chopper and the load to smooth the output from the chopper.

The DC bus voltage V1 may be provided from any suitable source, such as a battery, rectier circuit, fuel cell, or other unit. The filter generally has an inductor and a capacitor, and provides a DC voltage which is approximately equal to the average output voltage level of the chopper. This average output voltage Vo in turn is varied by changing the length of time for which the full DC bus voltage is passed to the lter, termed the on dwell time or pulse width of the chopper, or the length of time in which zero voltage is applied to the filter, termed the off dwell time of the chopper. Those skilled in the art will appreciate that this average output voltage Vn will also be a function of any variation or modulation of the input voltage level V1. That is, if any other system of energizing the input circut to the chopper is utilized in place of the two-level voltage arrangement just described wherein the full input voltage V, or zero input voltage is applied, then the output voltage Vo will be a function not only of the chopper on dwell time and ofr dwell time, but also of the modulation of the input voltage V1 applied to the chopper.

Various characteristics of the chopper system and its semiconductor components impose limits on the time durations of the on dwell time and the off dwell time of the chopper, in addition to limitations on the maximum 3,559,028 Patented Jan. 26, 1971 ICC chopper operating frequency. For example a certain minimum time is required for effective commutation of an SCR. Moreover rapid switching of a chopper circuit above a practical maximum produces overheating and consequent destruction of the semiconductors. With previous chopper systems it has been found that, particularly in high power chopper applications, in observing the maximum operating frequency and minimum pulse width limitations of the chopper it is not possible to provide an extended output voltage range without greatly oversizing the filter choke. As a corollary it has been found that the value of the volt-time integral applied to the filter choke, which is also a measure of the physical size and thus the cost of the filter choke, varies considerably at the different values of the average output voltage Vo produced by the chopper.

A salient aspect of the present invention is a method of operating a chopper system to provide a range of average output voltage V0 which extends virtually from zero to the full DC input voltage Vi. A corollary consideration of this invention is to provide a method of regulating the chopper which produces a minimum value of the volt-time integral over the output voltage range, thereby minimizing the physical size (and thus the cost) of the filter for al given chopper system.

SUMMARY OF THE INVENTION The present invention is useful with a DC-to-DC conversion system in which a load is energized through a lter at an output voltage level Vo determined by the chopper circuit. An input circuit supplies an input voltage or bus voltage V, to the chopper, and the on and off dwell times of the chopper itself are determined by signals received from a control circuit. Particularly in accordance with the method of this invention, the range of the output voltage Vo (that is, the percentage of input voltage Vi) passed to the load is regulated by establishing delivery of an output voltage Vo equal to one-half the input voltage V1 at substantially the maximum operating frequency of the chopper circuit. To deliver an output voltage level Vo less than one-half V, the chopper operating frequency is gradually decreased while maintaining the pulse width or on dwell time of the chopper substantially constant or allowing the pulse width to decrease. Similarly to deliver an output voltage Vo greater than one-half the input voltage Vi, the chopper frequency is gradually decreased from its maximum value as the chopper off dwell time is maintained substantially constant or allowed to decrease.

THE DRAWINGS In the several figures of the drawings like reference numerals identify like elements, and in those drawings:

FIG. l is a block diagram, partly in schematic form, of a high power DC-to-DC converter useful for practicing the inventive method;

FIGS. 2 and 3 are propaedeutic illustrations useful in understanding chopper operation;

FIGS. 4 and 5 are graphical representations useful in understanding various characteristics of chopper systems;

FIG. 6 illustrates waveforms of a Iknown method of chopper operation;

FIGS. 7 and 8 depict waveforms illustrating chopper operation in accordance with the method of this invention;

FIGS. 9 and 10 are graphical illustrations useful in understanding the method of this invention;

FIG. 1l illustrates waveforms denoting chopper operation in a method which is a variation of the method of this invention;

FIG. l2 is a graphical illustration contrasting opera# tion in accordance with the inventive method with other methods of chopper operation;

FIG. 13 is a block diagram of a control circuit useful in practicing the method of this invention;

FIGS. 14 and 15 are schematic diagrams depicting details of the control circuit shown generally in FIG. 13; and

IFIG. 16 is an equivalent circuit, and FIG. 17 illustrates waveforms, useful in understanding opeartion of the control circuit shown in FIGS. 13-l5.

GENERAL CHOPPER OPERATION FIG. 1 shows a chopper or converter 20 which includes a pair of switches 21, 22. For illustrative purposes these switches are depicted as mechanical units, but those skilled in the art will appreciate that in general the switches may be semiconductor units such as transistors, power transistors, or silicon controlled rectiers. A DC source 18 is connected to supply a voltage Vi over input conductors 23a, 23b to chopper 20. A battery, fuel cell, rectifier circuit or other energy source can provide the energy for application over conductors 23a and Z3b to the chopper. A control circuit 24 includes an adjusting mean 25 for regulating the timing and the duration of control signals applied over lines 26 and 27 to the switches 21, 22, respectively. Lines 26, 27 are depicted as mechanical linkages to close the mechanical switches 21, 22 but of course in a conventional circuit these lines represent the electrical conductors which pass the gating signals to the base or gate of the semiconductor unit.

A lter 28 is coupled between the output circuit of chopper 20 and the load to smooth the output voltage Vo passed to the load. As shown the lter comprises an inductor 30 coupled in series with one of the load conductors, and a capacitor 31 connected across the two load conductors to flter the output voltage. The voltage Vo is equal to the average output voltage of the chopper circuit 20, neglecting the series resistance of the filter.

The switches 21, 22 are connected such that when one switch is opening, the other switch is always closed. Thus when switch 21 is closed the full input voltage V1 is applied to the chopper and when switch 21 is opened this voltage application is interrupted. Switch 22 provides a DC connection between the choke and the input circuit during the off dwell time of the chopper. Such a connection obviates high voltage levels which would otherwise occur with the large change in current over a very small time span if switch 21 were opened without providing some other DC path. Of course a diode could be employed in place of switch 22 but generally a semiconductor switch is utilized in this positoin and thus this arrangement has been selected to depict the basic chopper operation.

FIG. 2 illustrates, with an idealized waveform, the operation of the chopper circuit. The input voltage level V, is shown at the left side, on the ordinate scale. At time t the system is at rest, and at time t1 switch 21 closes and the applied voltage passed to the :filter goes to the maximum level. At time t2 switch 21 is opened and switch 22 is simultaneously closed, so that the applied voltage decreases to zero. Thus the time interval between t1 and t2 denotes the pulse Width or the on dwell time of the chopper, which time is also referenced by A to facilitate` discussion hereafter. The chopper remains in this same condition until time t3 when, under the impetus of the signals applied-over lines 26, 27, switch 21 is again closed and switch 22 is opened. Thus this time period between t2 and t3 is the off dwell time vof the chopper. The total time period including the sum of the on and olf times is denoted T, both in FIG. 2 and hereafter in this explanation. Accordingly the off dwell time is equal to (T-A).

With the chopper operating as shown in FIG. 2, the average output voltage Vve is depicted by the dashed line, being equal to VQ. This. value is equal t0 The better to portray the advantages achieved by construction and operation of this invention, previous chopper or converter systems and their attendant disadvantages will be described. In large measure the signicant dilference between the system of this invention and previous chopper arrangements resides in the timing of the switching signals applied from the control circuit to the chopper. -A preferred circuit for deriving and applying the switching signals will be described hereinafter. Different nomenclature has been applied to the various known systems and methods for providing and applying the timing signals to the chopper. Each of these methods or systems constitutes a somewhat different arrangement which has its own distinctive characteristics. In describing and comparing these various systems, consideration will been given to the minimum on and ol dwell times of the chopper, the maximum operating frequency of the chopper, the range of the average chopper output volttage Vo, and to the physical size of the choke or inductor employed in the lter. For simplicity of explanation the average chopper output voltage Vo will be considered to be symmetrical about a value of (LSV, throughout the description; for example, 0.1V1 to A0.9V1, 0.3V, to 0.7V1, and so forth.

Because a primary consideration of this invention is the provision of a chopper control system in which the physical size of the inductor in the lter is minimized for a certain output voltage range, considering the maximum operating frequency fmax and the minimum pulsewidth A of the chopper, it will be helpful to have a general expression for the voltage-time integral of the filter choke. Such an expression can readily be evaluated andprovide an index of the physical size of the filter choke. By integrating the instantaneous value of the voltage appearing across the choke with respect to time, from the start of the period T until the termination of the on time A, or integrating from the end of the on time A to the expiration of the period T, the result is:

V* ik? 1) Because the lter choke must be sized to withstand the maximum value of the volt-time integral obtained by Equation 1, the maximum value of this expression will be significant during the following explanation.

PULSE-WIDTH MODULATION In the pulse-width modulation system for chopper control the operating frequency f, and thus the period T, remain xed, and the on dwell time A of the chopper is varied to provide control of the amplitude of the output voltage Vo. This operation is shown in FIG. 3 wherein the on dwell time A is gradually varied from a minimum value to a maximum value. The minimum on time .A is shown in FIG. 3a and the minimum of time (T-A) is shown in FIG. 3d. These minimum values are limitations which are determined primarily by the chopper components. Each semiconductor switch has limitations as to turn-on and turn-off times, and this imposes limitations of the volt-time integral given by Equation 1 changes.

These changes in the values of A and T-A are readily discernible in FIG. 3, but for a better understanding of the resultant changes in the value of the volt-time integral, reference is now |made to FIG. 4.

In FIG. 4 the values of T, the complete period of operation of the chopper, are plotted in milliseconds along the abcissa. The values of A, the time duration of the on cycle of the chopper, are plotted along the ordinate in milliseconds. A Ifamily of nine solid-line curves Kl-Kg of parabolic form are plotted for different values of constant volt-time integral, or constant physical size of the choke. Another family of curves is illustrated in broken line form, each being a straight line, plotted to represent constant values of A/T. In the pulse-width modulation system, T is maintained constant and A is varied. Accordingly in the illustration of FIG. 4, as the value of the average output voltage Vo is changed, the required combinations of A and T are found along a straight vertical line of constant T and variable A.

By way of example suppose that the maximum frequency fmax of the chopper were 200 hertz, the minimum value of T were l millisecond, and that a 9:1 voltage range (0.1Vi to 0.9V1) were desired. For the lower limit of this voltage range, A equals l millisecond and T equals 10 milliseconds; for this point 32 the corresponding value of the volt-time integral lies between K1 and K2. As the on time A increases, the value of A/ T similarly increases. At the mid-point of the voltage range, point 33, where A/ T equals one-half, the value of the corresponding volttime integral is K5. Thereafter the value of the volt-time integral decreases as the value of A/ T increases beyond 1/2.

FIG. 5 illustrates three different curves each depicting the results achieved with a different technique of modulation. Along the abcissa scale the output voltage VD is depicted as a percentage of the input voltage V1. On the ordinate scale the relative value of the volt-time integral, or absolute magnitude of the filter choke required, is represented. It will become apparent that operation over an output voltage range is represented -by a volt-time curve such as one of the three solid line curves depicted in FIG. 5. The maximum value of such a curve is of interest because it denotes the maximum value of the volt-time integral and therefore determines the size of the lter choke required for such a system. Curve 34 illustrates the values of the volt-time integral as a function of the average output voltage Vo of the chopper when pulsewidth modulation is utilized. It is noted that the maximum value of the volt-time integral for this system occurs at point 35 where A/ T equals 1/2.

There are several disadvantages inherent in the pulsewidth modulation syste-m. To reduce the maximum value of the volt-time integral or the maximum physical size of the choke (equivalent to passing through curves of smaller K values on FIG. 4), it is necessary to decrease the length of the period T. This however increases the operating frequency of the chopper and decreases the available range of the average output voltage V0, neither of which effects is desirable. To increase the average output voltage Vo it is necessary to increase T. However this in turn results in a larger value for the maximum volttime integral, or a larger physical dimension for the choke in the lter.- Accordingly it is manifestly difficult to observe the limitations of the chopper in the pulse-width modulation system, to maintain a relatively small volttime integral and still obtain the desired voltage range of the output voltage V0.

CONSTANT PULSE-WIDTH, VARIABLE- FREQUENCY MODULATION FIG. 6 illustrates variations in the chopper wave-forms for the constant pulse-width, variable-frequency modulation system as the on dwell time A of the chopper is held constant and the frequency or period T of the chopper is varied. The output voltage V0 from the chopper can thus be varied as shown in this illustration. Considering again the families of curves shown in FIG. 4, operation of the constant pulse-width, variable-frequency modulation system results in movement along a horizontal line, for which A has a constant value, as the value of T is varied. To correlate this system with the one just described, again the chopper operation is considered wherein the maximum frequency equals 200 hertz, minimum time T equals 1 millisecond, and a 9:1 voltage range is desired. To produce an output voltage Vo equal to 0.9Vi, operation at this point referenced 36 in FIG. 4 is indicated, where A equals 9 milliseconds and T equals l0 milliseconds. At point 36 the value of the volt-time integral, and thus the physical size of the choke, lies between the curves K1 and K2. As the output voltage Vo is decreased lfrom 0.9Vi to 0.1Vi (by holding the Value of A constant and increasing the value of T), the corresponding value of the volttime integral continues to increase. Curve 37 in FIG. 5 depicts a plot of the volt-time integral value against the average output voltage Vo -for the fixed pulse-width, variable frequency modulation system. It is particularly noted that the volt-time integral increases linearly as the average chopper output voltage Vo is decreased.

There are two major disadvantages with this second system. First, to obtain the desired voltage V0 at the lower end of the output voltage range, it is generally true that large values of the volt-time integral must be applied to the filter choke. Thus the choke must be greatly oversized to meet the lower output voltage requirement. Second, to obtain average output voltage values approaching Vi, the minimum oif dwell time (T-A) must be small, or else the period T must be large, which requires that still larger volt-time integral values must be applied to the lter choke at the lower end of the voltage range.

SYSTEM OF THIS INVENTION Maximum chopper frequency at V :1/2V1 Particularly in accordance with the present inventive method, an arrangement is provided in which both the pulse width A and the frequency (and thus the period T) are varied to achieve optimum system operation with a minimal physical size of the choke. For this arrangement it is initially assumed that the maximum operating frequency fmax of the chopper is limited only by the minimum pulse-width A of the chopper. The inventive method includes establishing the middle of the output voltage range, where Vo equals one-half Vi, at the point where the maximum chopper operating frequency occurs. This can be accomplished, by way of example, by producing a portion of the output voltage range of Vo with an on dwell time A of the chopper held constant and the frequency varied, and producing the remainder of the voltage range VD by maintaining the off dwell time (T -A) fixed and varying the frequency of the chopper. Such operation is illustrated by the waveforms depicted in FIGS. 7a-7g, inclusive. To obtain an output voltage Vo of 0.5V1, the value of A/ T is equal to 1/2, and the chopper is operating at its maximum requency f5 as shown in FIG. 7d. In this explanation subscripts of higher `value denote frequencies of greater magnitude; for example, f5 is higher than f4. To decrease the average chopper output voltage below the 0.5V1 value, the chopper frequency is gradually decreased through the frequencies f4, f2 and f1 as shown in the waveforms of FIGS. 7c, 7b and 7a. In this decrease of the output voltage Vo the on or dwell time A of the chopper is held constant as the frequency of chopper operation is decreased.

Conversely to increase the output voltage V., above the level of 0.5V1, the otf dwell time (T -A) is held fixed and the frequency of the chopper is varied as shown successively in FIGS. 7e, 7f and 7g. The frequencies are again designated f4, f2 and f1, representing frequency values successively less than the maximum frequency f5 shown in FIG. 7d. The better to illustrate the result of this system operation, reference is again made to FIG. 4.

It is assumed that fmax equals 200 hertz, minimum time T equals 1 millisecond, and a 9:1 voltage range is desired. When the chopper output voltage Vo is half the input voltage V1, the on dwell time A is equal to 1/2 of T. At point 39 in FIG. 4, by way of example, A is equal to half of T. To reduce the output voltage VD below this level, A is held constant and T is increased as shown in FIGS. 7c, 7b and 7a. In FIG. 4 this corresponds to movement from point 39 along a horizontal line to the right. It is manifest from Equation 1 that for values of output voltages Vo decreasing below 0.5V1, the value of the volttime integral increases. To increase the output voltage V above 0.5V1, as shown in FIGS. 7e, 7f and 7g, the off dwell time (T-A) is held consstant and T is increased. In FIG. 4 this voltage increase would be indicated by a diagonal line extending from point 39 upwardly to the right along a line whose slope is greater than the 0.5 con stant A/ T line, toward the upper right-hand corner of the graph across the curves of increasing K values. The value of the volt-time integral also increases under these conditions for values of Vo greater than 0.5V1. This increase of the value of the volt-time integral is shown by curve 38 in FIG. 5. The initial downwardly-sloping (relative to the horizontal axis) portion of curve 38 signifies a gradually decreasing value of the volt-time integral as the output voltage Vo goes from 0.1V, to 0.5Vi, with the lowest value of the curve occurring in the middle of the voltage output range. The value of the volt-time integral thereafter gradually increases as shown by the upwardly-sloping right hand portion of curve 38, in the output voltage range of 0.5V1 to 0.9Vi. Thus a characteristic V-shaped volt-time integral curve 38 is obtained with the variable pulse-width, variable frequency modulation system.

This system has several desirable features. In the first place the last described system readily provides a virtually unlimited range of output voltage Vo. By increasing the period T shown in FIG. 7a, the output voltage Vo of the chopper can be reduced to nearly zero. Likewise by increasing the period T shown in FIG. 7g the output voltage V0 can be increased to approximately the full input voltage V1. The minimum chopper pulse width and filter choke size considerations do not impose a stringent limitation on the maximum voltage range which can be achieved with the variable pulse-width, variable frequency system and this is a significant advantage over the earlier described systems.

In the second place the magnitude of the volt-time integral applied to the filter choke in the variable pulsewidth, variable frequency system represents another important advantage. As noted above it was assumed that the chopper can operate up to a frequency fmax limited solely by the minimum pulse-width A of the chopper. This point of maximum frequency corresponds to operation at the middle of the output voltage range as shown in FIG. 7d, and from inspection it is apparent that:

fmax=1/2Tmin (2) The better to illustrate the advantage of the value of the volt-time integral realized with this system, a typical set of conditions will be considered. It is assumed that the maximum value of the output voltage Vo is required to be 0.9V. To provide this output voltage with the smallest value of the volt-time integral, the waveform of the chopper voltage is identical for all the three described chopper modulation systems. Note the constant frequency chopper operation shown in FIG. 3d; the constant pulsewidth operation depicted in FIG. 6d; and the variable pulse-width, variable frequency operation illustrated in FIG. 7g. Accordingly the volt-time integral applied to the filter choke for each of these modulation arrangements is identical at the point where V0 equals 0.9V1, and this point is designated in FIG. 5.

Note, however, that for the pulse-width modulation arrangement described in connection with FIG. 3, as A is gradually decreased as shown in FIGS. 3c, 3b and 3a, the value of the volt-time integral increases as shown by curve 34 toward a maximum value at point 35, the mid-range point of the chopper output voltage level. Again, for the variable frequency modulation system described in connection with FIG. 6, as the frequency is gradually decreased to decrease the output voltage level as shown in FIGS. 6c, 6b and 6a, the value of the volt-time integral likewise increases as represented by curve 37 in FIG. 5. In contradistinction to the volt-time integral values for these two systems, curve 38 denotes that the volt-time integral gradually decrease for the variable pulse-width, variable frequency system as the output voltage Vo decreases from the maximum value at point 40. Even though the value of the volt-time integral again increases over the lower voltage range of the chopper, as shown by the left side of curve 38, the value of the volt-time integral never exceeds the value established at point 40 representing the other end of the range of output voltage, where the values of the volt-time integral for all three systems are the same. The earlier-described systems both increase the value of the volt-time integral substantially above the level at point 40 as the chopper output voltage level is decreased. Therefore each of the variable pulse- Width and constant pulse-width, variable frequency systems have a peak value of the volt-time integral substantially larger than the peak value in the variable pulsewidth, variable frequency modulation system. This graphic explanation of the advantages of the third system is readily confirmed by quantitive analysis of Equation 1 given above for each of the three modulation systems.

In the subsequent discussion the fixed pulse-width, variable frequency system described in connection with FIG. 6 and shown by the curve 37 in FIG. 5 will be dropped from consideration. Referring to FIGS. 3d and 6d, for which the values of A, T and Tmm are assumed identical, the volt-time integral values for the two systems are the same at 0.9Vi, represented by point 40 in FIG. 5. To reduce the output voltage V0 to 05V, with the pulsewidth modulation arrangement, the period T is held fixed and the on dwell time A is reduced to provide the maximum value of the volt-time integral at point 35 as shown in FIG. 5, where the output voltage is equal to 0.5Vi. However with the fixed pulse-width, variable frequency modulation system the value of the volt-time integral constantly increase, as depicted by curve 37, as the value of the output voltage decreases, with an increase in the period T as the on dwell time A is held constant. By evaluation for the corresponding values for the volt-time integral expression given in Equation l, it is demonstrable that under any conditions the pulse-width modulation technique always has a smaller peak value of the volttime integral. Accordingly the fixed pulse-width or fixed on dwell time A, variable frequency modulation system described in connection with FIG. 6 and represented by the curve 37 in FIG. 5 will no longer be considered.

HYBRID MODULATION SYSTEM Improvement of the variable pulse-width, variable frequency system It was assumed in describing the variable pulse-width, variable frequency system in connection with FIG. 7 that the only limitation on the maximum operating frequency of the chopper was the minimum chopper pulse-width A, in accordance with Equation 2. In actual practice however these theoretical limitations are not necessarily the true limitations on the system. A chopper used in high power applications may not be capable of operating at a frequency as high as that indicated by equation 2 for various reasons, such as over-heating of the components in the chopper. In other words as a practical matter the true limitation on the maximum operating frequency fmax of the chopper is not equal to the reciprocal of twice Tmm as indicated in Equation 2, but the maximum operating frequency is lower than this theoretical value. It is thus important to this invention to provide an improved operating method in which the chopper is operated in a more efficient manner than that of the variable pulsewidth, variable frequency system, and this improved system is termed the hybrid modulation system. This hybrid system assures that the advantage of reduction in the Volt-time integral value is obtainable even when the maximum operating frequency of the chopper is less than 1/ ZTmm.

Even though these practical considerations may impose some limitation on the maximum operating frequency of the chopper, nevertheless the variable pulse-width, variable frequency modulation system described above manifestly exhibits an advantageous volt-time integral characteristic curve (38, FIG. in the region where the chopper operating frequency is below its maximum value. Another way of viewing this frequency reduction is in connection with FIGS. 7a-7g. If it is assumed that the chopper cannot operate at the maximum frequency f5 indicated in FIG. 7d, the chopper can still operate at the considerably lower frequencies f4, f2 and f1 shown in this figure. With continued operation at the lower frequencies, the volt-time integral curve 38 is still correct for the upper (maximum V0) and lower (minimum Vo) portions of the output voltage range, but is no longer valid for the center portion near the value of 0.5V1.

Particularly in accordance with the present invention, effective control over the entire range can still be achieved at the lower maximum frequency by applying pulsewidth modulation at the maximum operating frequency of the chopper. Waveforms illustrating this operation are set out in FIGS. Saz-8h. Note that the lower portion of the output Voltage range is still achieved by operation similar to that of the system described in connection with FIG. 7. In FIGS. 8a and 8b, at the lower frequencies f1 and f2, the minimum pulse-width A is maintained and the period T is gradually reduced (T1, T2) to correspondingly effect the gradual increase in the output voltage V0. Particularly in accordance with the present invention, to enhance the chopper operation with the hybrid system, at frequency f3 pulse-width modulation is utilized to effect regulation of the amplitude of Vo over the central por,- tion of the output voltage range at one constant frequency f3, or one constant period T3, as depicted in FIGS. Sc-Sf. Thereafter, at the upper end of the output voltage range as shown in FIGS. 8g and 8h, the minimum off dwell time (T A) is maintained constant as the frequency is thereafter reduced, and T is increased, to approach the maximum voltages Vo of the chopper.

Referring again to FIG. 4, the just-described hybrid operation with pulse-width modulation at the maximum chopper frequency will result in a different movement of the value of the volt-time integral in this graph. With respect to FIGS. Sa and 8b, as the frequency is gradually increased Width the on dwell time A maintained constant, this is represented in FIG. 4 by movement from right to left along a horizontal line as T is reduced and A is maintained constant. When the maximum chopper frequency is reached, movement to the left along the horizontal line in FIG. 4 ceases. Then, as shown in FIGS. 8c-8f, the frequency remains constant, T remains constant, and A is gradually increased to provide regulation of Vo over the central portion of the chopper output voltage range. In FIG. 4 this corresponds to movement along a vertical line upwardly as the middle frequency is maintained constant. Thereafter as depicted in FI. 8g and 8h, the off dwell time (T-A) is maintained constant while A and T are gradually increased with decreasing frequency to approach the maximum value of output voltage V0. In FIG. 4 this corresponds to movement upwardly and to the right along a diagonal line. Those skilled in the art will appreciate that this movement along three straight lines, first the horizontal line to the left, then a vertical line upwardly, and then another straight line upwardly to the right, is a rough approximation of a constant volt-time integral curve such as the series of curves shown in solid lines in the drawing.

The more closely these curves can be approximated, the more nearly does the actual physical size of the choke required for the filter correspond to the minimum size that can be effectively used for a chopper when all the system parameters and operating conditions are considered.

FIG. 9 depicts three different volt-time integral curves for the hybrid modulation arrangement just described. Curve ABC is identical to the variable pulse-width, variable frequency modulation curve 38 shown in FIG. 5, and the broken line curve 34 represents the pulse-width Inoduation values consonant `with the showing in FIG. 5. If the theoretical maximum operating frequency were fmaxzl/ZTmm, the system could operate in the variable pulse-width, variable frequency modulation mode along the curve ABC. In accordance with the inventive teaching, assuming that a lesser value of the maximum chopper frequency fmaxl is the practical limitation on the system, the value of the volt-time integral is depicted in FIG. 9 by the curve which extends between points A and D, with pulse-width A held constant and T decreasing as shown in FIGS. 8a and 8b; over the pulsewidth modulation portion 41 to point E, as T is held constant and A is increased (FIGS. 8c-8f); and from lpoint E to point C, with (T-A) held constant and both A and T increasing as shown in FIGS. 8g and 8h. Similarly if the limitation on the maximum operating frequency is reduced further to the value fmaxz, the value of the volt-time integral is depicted by the curve A-F-42-G-C. A still further frequency decrease, to fmX3 for the maximum chopper operating frequency, results in a volt-time integral value ranging over the curve A-H-43-J-C. Thus the initial and terminal portions of the volt-time integral curve for the hybrid modulation system vary along the generally V-shaped curve characteristic of the variable pulse-width, variable frequency modulation curve 38, and the central portions 41-43 of each of the hybrid modulation curves vary in a manner similar to the characteristic 34 of the pulse-width modulation system.

Manifestly the hybrid operation system represents a substantial advance over the pulse-width modulation system. Note for example that reducing the maximum operating frequency of the chopper from fmax to fmax, provides a resultant curve in which the central portion 41 does not exceed the higher values at points A and C of the volt-time integral characteristic curve. In the limiting situation where the limitation of the maximum chopper operating frequency approaches lower and lower values, the characteristic curve for the hybrid system approaches that of the pulse-width modulation curve 34. In the range of practical chopper operating frequencies and the desired range of output voltage Vo, the hybrid modulation system has proved the most effective by reason of limiting the volt-time integral value, and thus the physical size and cost of the choke, provided in the filter.

A main consideration of the present invention is to minimize the physical size of the filter choke, analogous to the smallest value of the peak volt-time integral, for a given range of the output voltage V0. By way of example, the system characteristic for the fmax2 curve A-F-42-G-C shown in FIG. 9 will be considered, and this curve is reproduced in FIG. 10. It is noted that the peak value of the volt-time integral curves occurs at the midpoint of the output voltage range, at a value equal to 0.5V,. Accordingly if the system operates such that for other values of the output voltage V0, the level of the volt-time integral does not exceed this amplitude at the point 0.5V1, then operation of the system will still be acceptable. In other words, if the broken line 45 in FIG. 10 is taken as the limiting condition, any modification of the chopper modulation method which still produces a volt-time integral curve of an amplitude not exceeding that of curve 45 will still permit chopper operation with the same size filter choke. If the volt-time integral curve is varied, as shown by either of the broken line curves 46 and 47 in FIG. 10, this operation would still be acceptable. These curves 46, 47 could in fact be produced by concomitant variation of both the pulsewidth A and the period T over the entire range of output voltage V0, in a manner illustrated in FIGS. 11m-11g. Operation such as that represented by the waveforms of FIG. 11 produces combinations of A and T, on the graphical representation of FIG. 4, which define a path tending to follow more closely the parabolic constant volt-time integral curves K1-K2. To vary the amplitude of the output voltage Vo from some given output voltage corresponding to a certain point A1, T1 on FIG. 4 to some different output voltage Vo represented by a second point A2, T2, the selected combinations of A and T define a curve which does not cross any of the constant volttime integral curves Kl-Kg of a value larger than the value associated with either of the points A1, T1 and A2, T2. It is again emphasized that these two points have been initially selected to provide the smallest volttime integral value possible, thus minimizing the physical size A( and cost) ofthe filter choke.

FIG. 12 is a pictorial representation of the operating advantages achieved by the present invention with the hybrid modulation system. In an illustrative example let Tmm equal 1 millisecond, fmax equal 200 hertz, and let a 9:'1 voltage range be produced. The curve 37 shows the smallest value possible for the volt-time integral for the xed pulse-width, variable frequency modulation systern. Curve 34 depicts the minimum value obtainable for the volt-time integral value characteristic in the variable pulse-width, constant frequency modulation system. The variable pulse-Width, variable frequency modulation arrangement produces a range of values depicted by curve 52. In accordance with the inventive teaching the hybrid modulation system provides a volt-time integral characteristic curve always below the line 53 which corresponds to one of the solid line curves in FIG. 4. The actual value of the curve may lie anywhere in the shaded area 54. Shown in broken line 55 is a typical volt-time integral curve in the hybrid modulation range obtained when A is varied continuously as shown in FIG. 11. The solid line curve S6 shows the hybrid modulation technique in which the system is first operated with a decreasing T as shown in FIGS. 8a, y8b. At the maximum frequency, T is held constant and A is varied as shown in FIGS. '8c-8f, and thereafter (T-A) is held constant as A and T are gradually increased to the upper limit of the output voltage V0. It is thus manifest that the minimum physical size of the filter choke for the given range of output voltage Vo is obtained with the hybrid modulation system.

DESCRIPTION OF CONTROL CIRCUIT 24 In FIG. 1 control circuit 24 is represented generally as an adjustable unit for passing control signals over lines 26, 27 to regulate operation of the chopper switches 21, 22. To implement the practice of the hybrid modulation system described above, a suitable control circuit 24 will now be described.

FIG. 13 depicts in block form the general circuits of the complete control circuit system. In unit 60 is any suitable circuit for generating a controlled, variable input signal for application over line 61 to compensating circuit 62. In turn the output signal from circuit 62 is passed over line 63 to a difference amplifier 64, which provides a pair of output signals designated I1 and I2. These two signals are applied over lines 65 and 66 to a pair of stages 67, 68, each of which includes sensing and clamping circuits. The outputs from these two stages are respectively applied over lines 70, 71 to a pair of pulse circuits 72, 73, which in turn apply their output signals over lines 74, 75 to a flip-flop or multivibrator stage 76. The state of the tiip-iiop 76 at any time is denoted by signals 12 fed back from stage 76 over lines 77 and 78 to the respective sensing and clamping circuits 67, 68. Output signals from the fiip-iiop stage are passed over lines 26, 27 to provide signals for firing the semiconductor switches in a chopper circuit, thus to provide regulation of the chopper on time and chopper off time.

The more detailed description of the control circuit shown generally in'FIG. 13 is conveniently divisible into two portions. The initial description will include the input portion of the circuit, from the receipt or provision of the adjustable control signal in stage 60 to the generation of the two difference signals I1 and I2. Details of this portion of the circuit are depicted in FIG. 14.

The compensating circuit is in the upper portion of FIG. '14 and includes a pair of NPN type transistors 80 and 81. The lower portion of FIG. 14 includes the difference amplifier with output conductors 65, 66 for passing signals I1 and I2 to the sensing and clamping circuits. A pair of PNP type transistors 82, 83 are included in the difference amplifier. Those skilled in the art Will appreciate that the specific components illustrated are by way of illustrating actual operation of a preferred embodiment. Transistors of one type can readily be replaced by transistors ofthe opposite type, with the accompanying polarity reversal of the supply voltages and the signals.

A pair of input terminals 84, 85 are provided in FIG. 14, and terminal 85 is connected to ground conductor 86. Terminal 84 is coupled over conductor 87 to the base of transistor 80. The circuit of FIG. 14 is energized by applying a suitable undirectional potential difference between terminal 93 and ground conductor 86, with the polarity at conductor 92 being positive relative to the ground conductor.

An input signal designated V2 is applied between terminals 84, 85, and thus this signal is applied to the base of transistor 80. Such input signal could also be obtained by suitable current adjusting means, for example, by a potentiometer. A reference voltage is established at the common connection between resistor 94 and a Zener diode 95, which circuit is coupled between conductor 92 and ground. The base of transistor 81 is coupled to the connection between resistor 94 and diode 95, and the collector of this transistor is coupled through a resistor 96 to conductor 92. The emitter of this transistor is coupled over a conductor 97 to the difference amplifier. Transistor 81 and its associated circuit provide compensation for changes in the base-emitter voltage drop from transistor 80 which may occur by reason of temperature variation.

The collector of transistor 80 is coupled through a resistor 98 to conductor 92. Transistor 80 essentially functions as an input resistor which limits loading of the input signal by the difference amplifier circuit. 'Ihe base emitter junctions of transistors 80, 81 are forward biased and their base-collector junctions are reverse biased. The voltage appearing at the emitter of transistor 80 and passed over conductor 100 to the difference amplifier circuit is equal to the input voltage Va minus the baseemitter voltage drop of transistor 80. Similarly the voltage established on conductor 97 is equal to the reference voltage between resistor 94 and diode 95 minus the baseemitter voltage drop of transistor 81.

A resistor 101 is coupled between conductor 100 and the emitter of transistor 82. Conductor 100 is also coupled to the top of a parallel circuit which includes a Zener diode 102 in one leg, and a potentiometer 103 seriescoupled with a resistor 104 in the other leg. A resistor 105 is coupled between the bottom of this parallel circuit and ground conductor 86. The movable arm of potentiometer 103 is coupled to the base of transistor 83, and the emitter of this transistor is coupled over a resistor 106 to conductor 97. Conductor 97 is also coupled to the top of another parallel circuit which includes a Zener diode 107 in one leg and a potentiometer 108 seriescoupled with a resistor 110 in the other leg. The bottom of this parallel circuit is coupled through a resistor 111 to ground conductor 86. In the difference amplifier circuit potentiometers 103, 108, resistors 104, 105, 110, 111 and Zener diodes 102, 107 provide adiustable bias voltages for the bases of transistors 82 and 83. The difference amplitier applies a first output signal I1 over conductor 65 to sensing and clamping circuit 67, and the other transistor 83 in the difference amplifier provides an output signal I2 over conductor 66 to the other sensing and clamping circuit 68.

FIG. 15 shows an energizing terminal 112 and a ground conductor 113, .which conductor can be connected to ground conductor 86 (FIG. 14) as the same unidirectional potential dilference applied to terminal 93 is also applied to terminal 112. The power supply for the circuit of FIG. 15 need not be well regulated but may fluctuate over a considerable range, by reason of the connection of resistor 114 and Zener diode 115 between terminal 112 and ground. The potential on conductor 116 is thus a well-regulated potential, and is positive relative to ground conductor 113. The components shown in FIG. 15 include the sensing and clamping circuits 67, `68, the pulse circuits 72, 73, and the flip-flop 76 shown more generally in FIG. 13. Because the circuit of FIG. 15 is symmetrical, a detailed description of half will suffice for a clear understanding of the entire circuit and its operation.

In the left portion of FIG. l is a unijunction transistor 117 having its emitter connection coupled through a resistor 118 to conductor 65, which conductor is also coupled through a capacitor 120 to ground conductor 113. The base-two connection of transistor 117 is coupled through a series circuit including a resistor 121, a diode 122, and another resistor 123 to conductor 116. The common connection between diode 122 and resistor 123 is coupled to the collector of NPN type transistor 124 in the circuit of flip-flop 76, and the emitter of this transistor is coupled directly to ground conductor 113.

The base-one connection of transistor 117 is coupled through a resistor 125 to the ground conductor 113, and both a diode 126 and primary winding 127 of a pulse transformer 128 are coupled in parallel with resistor 125. The secondary winding 130 of the pulse transformer has one side grounded, and the other end of the winding is coupled through a resistor 131 and a diode 132 to the base of transistor 124. A resistor 133 is coupled between the base and emitter of this same transistor, and the common connection between diode 132, resistor 133 and the base of transistor 124 is extended over a conductor 134 to one side of a parallel circuit including resistor 135 and capacitor 136. The other side of this parallel circuit is coupled to output conductor 27, the collector of the other NPN type transistor 137 in flip-flop 76, and through a resistor 138 to conductor 116.

Another parallel circuit comprises a resistor 140 and a capacitor 141, and the left side of this circuit is coupled to output conductor 26, resistor 123, diode 122, and the collector of transistor 124. The right side of parallel circuit 140, 141 is coupled over a conductor 142 to the common connection between a diode 143, a resistor 144, and the base of transistor 137. The other side of resistor 144 is coupled to ground conductor 113, and the other side of diode 143 is coupled through a resistor 145 and secondary winding 1'46 of pulse transformer 147 to ground. The primary winding 148 of this transformer has one side coupled to ground conductor 113, and diode 150 and resistor 151 are coupled in parallel with primary winding 148. The common connection between this primary winding, diode 150 and resistor 151 is coupled to the base-one connection of another unijunction transistor 152, of which the base-two connection is coupled through a series circuit including a resistor 153 and a diode 154 to the common connection between resistor 138, output conductor 27 and the other illustrated components. The emitter connection of transistor 152 is coupled through a resistor 155 to the common connection between input conductor 66 and a capacitor 156, with the other side of this capacitor being coupled directly to ground conductor 113.

Considering the operation of the circuit in FIG. l5 in general terms, charging current such as I1 received over input conductor 65 charges capacitor 120 and the voltage of this capacitor is applied through resistor 118 to the emitter of transistor 117. A bias voltage for the base-two connection is applied over diode 122 and resistor 121. When the charge accumulated in capacitor is suiciently high, transistor 117 conducts or fires and a pulse is applied-through this unijunction transistor and across the primary winding 127 of pulse transformer 128. As this pulse abruptly ceases to flow through the primary winding, diode 126 clamps the voltage at that level across the primary winding. The pulse is transmitted over secondary winding through resistor 131 and diode 132 to the base of transistor 124 to switch this transistor on and, through parallel circuit 140, 141 and conductor 142, to switch transistor 137 olf. Thus the state of flipeflop 76 is changed each time capacitor 120 accumulates a voltage suciently high to re transistor 117 and pass a pulse over transformer 128. Accordingly the output signals from conductors 26, 27 can be utilized to switch the semiconductor units in the chopper, to ultimately control the times of chopper operation and non-operation.

The operation of the circuits shown in both FIGS. 14 and l5 may be better understood by considering the equivalent circuit diagram of FIG. 16. As there shown a rst current generator is connected to provide charging current I1 over conductor `65, and another current generator 161 provides the other charging current I2 over conductor 66. A capacitor C1 is coupled in parallel with a switch S1 between current generator 160 and ground conductor 162. Similarly another capacitor C2 is coupled in parallel with a second switch S2 between current generator 161 and ground conductor 162. The purpose of the equivalent circuit is to generate two timing signals denoted t1 and t2 in FIG. 17a, each of variable duration, to provide timing signals for the hybrid modulation system for controlling the on and olf dwell times of the semiconductor switches referenced generally as 21, 22 in FIG. 1. These two semiconductors are always in opposite states, as previously explained. Operation of the equivalent circuit illustrated in FIG. 16 will now be described.

It is initially considered that switch S1 is closed and S2 is open as shown in FIG. 16. Accordingly capacitor C1 is effectively short circuited and all of the current I1 from the generator 160 flows through S1. This current is referenced Isl in FIG. 16 and depicted as the waveform in FIG. 17b. Thus at this time there is no charge being accumulated in C1, and the voltage V1 across this capacitor is zero as shown in FIG. 17C. At the same instant switch S2 is open and current I2 from generator 161 is thus linearly charging capacitor C2. The absence of any current flow 1.52 through switch S2 at this time is indicated by the zero level of the initial portion of the waveform depicted in FIG. 17d, and the linear charge accumulation across capacitor C2 to provide the voltage V2 thereacross is indicated in FIG. 17e, where the voltage rises from zero to a peak value Vp.

When the voltage accumulated across C2 reaches the peak value Vp, switch S2 closes and switch S1 opens. When switch S2 closes capacitor C2 discharges through S2, as evident in FIG. 17e, and the charging current I2 from generator 161 bypasses capacitor C2 and flows through S2, as shown in FIG. 17d. As switch S1 opens, current Isl no longer flows through the switch (FIG. 17b) but instead begins a linear charge of capacitor C1 to provide voltage V1 thereacross (FIG. 17e). When the voltage across C1 reaches the peak value Vp, switch S1 closes and switch S2 opens. Capacitor C1 then discharges through S1 and the charging current I1 is also bypassed through this closed switch. This cyclical'operation continues as indicated in the idealized waveforms of FIGS. 17b417e. The timing signals referenced t1 and r2 in FIG. 17a correspond to the respective charging times of capacitors C1 and C2 `as is manifest from FIGS. 17C and 17e.

With this explanation of the idealized circuit operation, the operation of difference amplifier 64 shown in FIG. 14 is more readily understood. In this schematic diagram, when Va at terminals 84, 85 equals the reference voltage established at the' base of compensating transistor 81, trimmer potentiometers 103, 108 are set so that the voltages applied tothe bases of transistors 83 and 82 are equal, and hence 11:12. If Va now increases, such voltage increase appears across resistor 105 andv hence the base bias voltage to transistor 83 is increased. The emitter current and thus the collector current of transistor 83 is therefore reduced. At this time, however, the voltage applied to the base of transistor 82 has not changed, Va has been increased, and thus the emitter and collector currents flowing through transistor 82 increase. The magnitude of this increase in output current I1 is equal to the amount of the decrease in output current I2 from transistor 83, and thus the sum of the currents I1 and I2 has not changed.

In an analogous manner if Va decreases, the voltage decrease appears across resistor 105 and thus at the base of transistor 83. It follows that the emitter and collector currents of this transistor increase, With the decrease of Va the emitter and collector currents of transistor 82 decrease. Once again the changes in currents Il and I2 are equal in magnitude but opposite in direction and thus the sum of the currents I1 and I2 has not changed.

lf the level of the input signal Va becomes sufiiciently high, the base-emitter junction of transistor 83 becomes reverse biased, I2 goes toward zero in magnitude, and the time t2 which is required to charge a capacitor with current I2 approaches infinity. Conversely as the amplitude of the input signal becomes sufficiently low, transistor 82 is cut off, I1 approaches Zero level and t1 approaches infinity. From the previous explanation of general chopper considerations, it is apparent that these limiting conditions are related to a chopper output voltage Vo of virtually zero V1, and to the other limiting condition where the chopper output voltage V0 approaches full Vi or the full DC bus voltage applied to the chopper.

In the schematic diagram of FIG. 15, currents Il and I2 generated in the manner just described flow over conductors 65 and 66 into this circuit. Considering initial conditions wherein transistor 124 of flip-flop 76 is on or conducting, the collector of transistor 124 is virtually at ground potential, and hence no bias voltage is applied to the base-two connection of unijunction transistor 117. All of the current I1 is thus flowing through resistor 118,

the emitter to base-one junction of transistor 117, and the saturated primary winding 127 of transformer 128. The collector voltage Vc of transistor 124 is shown in FIG. 17f, and the legend VX denotes the voltage level established by resistor 114 and Zener diode 115.

With transistor 124 on, transistor 137 is off or nonconducting and the collector potential Vc of this transistor is approximately VX as shown in FIG. 17g. A bias voltage is applied to the `base-two connection of unijunction transistor 152 through diode 154 and resistor 153 at this time. The emitter to base-one junction of transistor 152 is reverse-biased and thus current I2 is linearly charging capacitor 156. When the voltage across this capacitor reaches the peak emitter voltage of transistor 152, unijunction transistor 152 fires and a voltage pulse is applied across transformer 147 to the base of transistor 137 in ip-Jop 76. This pulse turns transistor 137 on which in turn switches olic transistor 124, changing the state of Hip-flop 76. The collector potential Vc of transistor 137 drops virtually toground potential (FIG. 17g) and the collector potential Qf tranSiStor 124 rises to approximately Vx (FIG. 17f). With `Vc of transistor 137 at nearly ground potential, no bias voltage is applied to the basetwo connection of unijunction transistor 152, and hence after capacitor 156 discharges through this unijunction transistor, current I2 continues to ow through resistor 1,55, the emitter to base-one junction of transistor 152, and the saturated primary lwinding 148 of transformer 147. At the same instant that ip-flop 76 changed states, the collector voltage Vc of transistor 124 rose to approximately VX, thus applying a bias voltage to the base-two connectionof unijunction transistor 117, reversebiasing the emitter to Ibase-one junction of this transistor. Accordingly the current I1 no longer flows through transistor 117 and'instead begins to charge capacitor 120 in the linear manner. From this description of a half-cycle of operationv of the ip-lop circuit, the subsequent halfcycle vand successive operation of the circuit is readily apparent.

'It is again emphasized that the operation of the circuits depicted in FIGS. 14 and 15 is inherently free of aberrations due to temperature variation. Flip-op 76 can be designed to operate over a wide range of temperatures. In addition, with proper design of the unijunction circuits, ,the voltage level at which the unijunction transistors fire can be established at a very stable level to provide a corresponding stability of the durations of the timing signals t1 and t2.

After the foregoing description of the interconnection and operation of the control circuit depicted in FIGS. 13- 16, it can readily be shown that the volt-time integrals which are present when the described control circuit is constructed and operated are precisely those which allow practice of the inventive method detailed in the earlier portion of the specification. It has been emphasized that the control circuit is designed so that the magnitudes of the currents I1 and I2 are variable, lbut the sum of the magnitudes of these two currents is always a constant value. For this demonstration let R represent this constant amplitude value. i

I1|12=R From-the foregoing explanation, and particularly in connection with the equivalent circuit of FIG. 16, it is manifest that I t1 ,and Igt2 (4) The term t1 used in connection with FIGS. 16 and 17 corresponds to the term A used with FIGS. 2-8, and similarly the term (t1-H2) corresponds to the term T. Thus with t1=A and t2=T-t1, it follows that VDC1+ VpC'z A T-A (6) By selecting the magnitude of C1 to be equal to C2, it follows that the products VpCl and VpC2 are constants, and are equal to each other. Letting S represent this constant value Because R and S are both constants, then the term A2 A-r for the described control circuit is also a constant. As noted above in `Equation 1, this term, in connection with the input voltage V1, is the generalized expression for the volt-time integral applied to the filter choke, when the described control circuit is utilized to open and close the semiconductor switches in the chopper. The circuitry thus presents a constant volt-time integral to the filter choke as represented by brokenline 45AinFIG.l .10. It follows that the described control circuit operates in one of the hybrid modulation technique modes explained earlier, and

thus operation of the chopper system with the minimum size of filter choke is attained.

Solely to assist those skilled in the art to` practice the method of` this invention, a table of component identifications and circuit values for the circuits of FIGS. 14 and 15 is set out below. This information is given solely to implement practice of the invention, and in no sense by way of limitation.

Identification of value Component:

80, 81, 124, 137 2N3642. 82, 83 2N3638. 117, 152 2N3'647. 95 IN4743. 102, 107 1N4738.` 115 1N740.

120, 156 1.27 afd. 136, 141 300 uuid. 103, 108 0-5`K ohms. 118, 155 22 ohms. 121,153 100 ohms. 12S, 151 12() ohms. 114, 123, 138 220 ohms. 96, 98 470 ohms. 105, 111 680 ohms. 1 01, 106 750 ohms. 94 Ohms. 104, 110 1.8K ohms. 131, 145 2.2K ohms. 133, 135, 140, 144 8.2K. ohms.

While Aonly particular methods ofv theinvention have been described, it will be apparent to those skilled inthe art Athat various changes'and modifications may be made therein without departing from the invention in itsI broader. i

the durations of the on and oif times, the method of regulating the range of output voltage passed to the load as a fraction of the input voltage passed to the chopper, comprising:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper; and delivering output voltage levels less than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while maintaining the chopper on dwell time approximately constant, to minimize the filter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency.

2. ln a DC-to-DC conversion system in which a load is energized through a filter which includes an inductor from a chopper circuit at a level determined by the on dwell time and olf dwell time of the semiconductor switch in the chopper circuit, with a control circuit passing regulating signals to the chopper circuit to regulate the durations of the on and off times, the method of regulating the range of output voltage passed to the load as a fraction of the input voltage passed to the chopper, comprising:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper, while employing pulse width modulation to gradually increase the on dwell time of the chopper frem substantially its minimum value to the point where the olf dwell time of the chopper is at substantially its minimum value; and

delivering output voltage levels less than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while maintaining the chopper on dwell time approximately constant to minimize the filter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency.

3. In a DC-to-DC conversion system in which a load is energized through a lter which includes an inductor from a chopper circuit at a level determined by the on dwell time and off dwell time of the semiconductor switch in the chopper circuit, with a control circuit passing regulating signals to the chopper circuit to regulate the durations of the on and off times, the method of regulat ing the range of output voltage passed to the load as a fraction of the input voltage passed to the chopper, comprising:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper; and delivering output voltage levels less than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while decreasing the on dwell time as the chopper operating frequency decreases, to minimize the lilter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency.r y

f' 4. In a DC-to-DC conversion system in which 'a load isl energizedl through a filter which Aincludes anv inductor from a chopper circuit at a level determined' by the onA dwellA time and off dwell time of the semiconductor switch in the chopper circuit, with a control circuit passing regulating signals to the chopper circuit to regulate the durations of the on and off times, the method of regulating the range of output voltage passed to the load as a fraction of the input voltage passed to the chopper, oomprising:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper, while employing pulse width modulation to gradually increase the on dwell time of the chopper from substantially its minimum value to the point where the off dwell time of the chopper is at substantially its minimum value; and delivering output voltage levels less than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while decreasing the on dwell time as the chopper operating frequency decreases, to minimize the lter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency.

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper; and

4delivering output voltage levels greater than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while maintaining the chopper otf dwell time approximately constant, to minimize the lter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency.

6. In a DC-to-DC conversion system in which a load is energized through a filter which includes an inductor from a chopper circuit at a level determined by the on dwell time and off dwell time of the semiconductor switch in the chopper circuit, with a control circuit passing regulating signals to the chopper circuit to regulate the durations of the on and 01T times, the method of regulating the range of output voltage passed to the load as a fraction of the input voltage passed to the chopper, comprismg:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper, while employing pulse width lmodulation to gradually increase the on dwell time of the chopper from substantially its minimum value to the point where the off dwell of the chopper is at substantially its minimum value; and

delivering output voltage levels greater than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while maintaining the chopper off dwell time approximately constant, to minimize the filter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency.

7. In a DC-to-DC conversion system in which a load is energized through a tilter which includes an inductor from a chopper circuit at a level determined by the on dwell time and olir dwell time of the semiconductor switch in the chopper circuit, with a control circuit passing regu- 20 lating signals to the chopper circuit to regulate the durations of the on and off times, the method of regulating the range of output voltage passed to the load as a fracion of the input voltage passed to the chopper, comprising:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper; and delivering output voltage levels greater than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from its maximum value, while decreasing the off dwell time as the chopper operating frequency decreases, to minimize the filter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency. 8. In a DC-to-DC conversion system in which a load is energized through a lter which includes an inductor from a chopper circuit at a level determined by the on dwell time and off dwell time of the semiconductor switch in the chopper circuit, with a control circuit passing regulating signals to the chopper circuit to regulate the durations of the on and off times, the method of regulating the range of output voltage passed to the load as a fraction of the input voltage passed to the chopper, comprising:

delivering an output voltage level equal to one-half the input voltage level at the maximum operating frequency of the chopper while employing pulse width modulation to gradually increase the on dwell time of the chopper from substantially its minimum value to the point where the off dwell time of the chopper is at substantially its minimum value; and

delivering output voltage levels greater than one-half the input voltage level to the chopper by gradually decreasing the chopper operating frequency from ,its maxamum value, while decreasing the off dwell time as the chopper operating frequency decreases, to minimize the lter inductor size for a given set of chopper operating characteristics such as output voltage range, minimum switching time and maximum operating frequency. i

References Cited UNITED STATES PATENTS 3,226,630 12/1965 Lampke S21-9A 3,360,712 12/1967 Morgan 321-43 WILLIAM M. SHOOP, JR., Primary Examiner U.S. Cl. X.R. 321-9, 43 

